Method for removing metal coatings from bases



Patented Apr..13, 1943 FROM BASES Earl C. Hall, Oak Park, 111., assignor to Western Electric Company, Incorporated, New York, N. Y., acorporation of New York No Drawing. Application April 10, 1940, Serial No. 328,879

Claims.

This invention relates to methods for removing metal coatings from a base. Many types of articles are provided with metal coatings, usually by electroplating, and. the nature of the coatings applied in each instance generally depends upon the service requirements of the article. For example, certain magnetic cores used in communication apparatus function in contact with other apparatus members and thus require a hard, particularly smooth and corrosion resistance surface in addition to good magnetic properties. Certain of these cores have been made by forming a base of magnetic iron or an iron bearing magnetic alloy. such as Permalloy,

electrodepositing a coating of nickel over the base,

and then plating a coating of chromium over the nickel. In these cores the relative weights and characteristics of the individual metal coatings are critical and it is sometimes necessary to either discard or reprocess large quantities of the cores after they have been plated due to defects in the coatings. cores it is desirable to provide some means for removing both the chromium and the nickel, rapidly, completely and inexpensively without a1- fecting the base material and the usual stripping methods do not meet these requirements adequately. Removal of the coatings by mechanical means, as by grinding, is costly and usually results in some damage to the base, particularly when the article has an irregular configuration. Separation of the coatings and base by the use of heat is not very practical because the spread between their respective expansion coefilclents is not sufliciently great to cause the plate to crack ofi when the coated base is heated to any reasonable temperature. The coatings can, of course. be dissolved in acids, such a hydrochloric acid for the chromium and concentrated nitric acid for the nickel. However, the action of these acids is slow and their use requires careful handling and control to prevent attack on the base material, and in general the use of these dissolving agents is not readily adapted to large scale manufacturing methods.

An object of this invention is the provision of improved and inexpensive methods foreconomically stripping a combination of metal coatings, such as a combination of nickel and chromium coatings, from a :ferrous composition base without afiecting the base.

In accordance with one embodiment of the invention, articles having a base of iron, or iron alloy, with an external coating of chromium over To justify reprocessing these defective an intermediate coating of nickel are immersed in a water solution of chromic acid and subjected to the action of direct current with the articles as anodes. Removal of both the chromium and nickel coatings proceeds rapidly with the flow of current and the base is passivated promptly on 'exposure to the solution and current. As the base is exposed to the solution, a smut is formed thereon which affords protection against atmospheric corrosion until it is removed with hydrochloric acid prior to the time the base is replated. i

The stripping operation can be performed conveniently in a conventional electroplating apparatus, comprising the usual tank for containing the electrolyte and electrodes mounted in a suitable direct current circuit. The articles are sus- In preparation of the electrolyte, chromium I trioxide (CrOa) is dissolved in water. Commercial chromic acid is suitable for this purpose.

The preferred solution contains approximately two pounds of chromium trioxide per gallon of water, although some modifications of these pro-- portions are permissible and concentrations ranging from one to four pounds of chromium trioxide per gallon of water can be used.

This solution of chromic acid in water gives good results for a relatively short period of service and is suitable for processing limited quantitles of articles. However, if the service life of this solution is greatly prolonged, the stripping action becomes retarded. At such times a quantity of boric acid (HaBOs) can be added to rejuvenate the solution and restore its efilciency. About three ounces of boric acid per gallon of solution is generally used and additions of between one ounce and five ounces per gallon of solution are efifective. However, the boric acid is preferably added at the time the solution is prepared and before it is used and by maintaining the boric acid content within the above range, the solution can be used efiiciently for a very long period and for processing large quantities of articles.

The temperature of the solution-with or without the boric acid addition should be held between F. and 185 F., preferably at about F.. for best results. This can be done conveniently by means of steam coils immersed in the solution.

The current density can be varied somewhat, but relatively high values are used preferably, particularly when boric acid is used in the electrolyte. Current densities between 150 an i 250 amperes per square foot of anode or article surface have been used and values around amperes per square foot give excellent results.

As soon as the current is applied to the immersed and anodically connected articles, removal of the coatings begins and continues at a rapid rate until the coatings are completely stripped from the base. The time required for complete stripping depends largely upon the weight of the coatings. Ferrous base articles having a milligram per square inch coating of nickel over the base and a 15 milligram per square inch coating of chromium over the nickel can be completely stripped in less than fifteen minutes at the above current densities and heavier or lighter coatings require proportionally more or less process time.

The stripping process with and without the boric acid in the electrolyte does not attack or damage the base of the articles, either when the base is made of iron or of certain ferrous alloys or compositions. The process can be used for stripping articles having a base of magnetic iron, or a base of nickel-iron magnetic alloy of the Permalloy series, such as an alloy, containing essentially nickel and the balance iron. In processing .both of these article types, the stripping action proceeds rapidly until the coatings are completely removed, and the base is passivated immediately and progressively by exposure to the action of the currentand electrolyte.

As rapidly as portions of the base are exposed a dull black deposit is formed thereon. The formation of this deposit, or smut, indicates that the stripping action has been completed and its appearance provides a simple means for regulating the process. This deposit also affords protection of the base against atmospheric corrosion after it is removed from the stripping solution.

The stripped articles can be stored if desired andthe smut removed by immersion in hydrochloric acid just before the articles are replated.

Because the stripping action does not damage the ferrous base, control of the process time is not critical. It is important only to leave the articles in the electrolyte until the coatings are completely removed, as evidenced by the black color of the articles, and further exposure to cur rent and electrolyte does no damage. Consequently, a large number of articles can be stripped at one time without the necessity for close and costly process control. After the stripping oper- .ation, the articles can be stored and replated by the usual methods, after a hydrochloric acid dip to remove the smut coating, without being subjected to any machining or polishing operations. The original contour and dimensions of the base are not affected and thus the properties of the replated article which depend upon these factors are fully preserved.

This process can be used advantageously for processing a variety of article types in'addition to the specific core structure described herein. For example, in some cases an undercoating of copper is employed in combination with nickel and chromium coatings and the process i very satisfactory for stripping this type of article. The copper is removed rapidly in the same manner as the nickel and chromium, and the stripped base is not damaged by the process. Also, the process can be employed for stripping a singl'e coating of any one of these metals and can be used with ferrous base materials other than magnetic iron and nickel-iron magnetic alloys.

It will be apparent that further modifications of the specific invention embodiment described herein are feasible, and it is to be understood that the invention is limited only by the scope of the appended claims.

What is claimed is:

1. In a method of removing a combination of nickel and chromium coatings from a nickeliron alloy base and simultaneously forming a protective smut coating on the base, the steps of placing the coated base in a water solution consisting essentially of 1 to 4 pounds of chromium trioxide and 1 to 5 ounces of boric acid per gallon of water in water, and passing a current through the solution with the base as an anode. 2. In a method of removing a combination of nickel and chromium coatings from a base comprising about 45% nickel and the balance iron, the steps of placing the coated base in an electrolyte consisting of water, one to four pounds of chromium trioxide per gallon of water, and from one to five ounces of boric acid per gallon of water, and passing an electric current from the coated base used as an anode to a cathode to remove the nickel and chromium coatings from the base and form a protective smut coating on the base.

3. In a method of removing a combination of nickel and chromium coatings from a ferrous composition base, the step of .anodically electrolyzing the coated base in a solution consisting essentially of l to 5 ounces of boric acid and 1 to 4 pounds of chromium trioxide per gallon of water while maintaining the temperature of the solution between 155 F. and 185 F.

4. In a method of removing a combination of nickel and chromium coatings from a ferrous composition base, the steps of immersing the coated base in an electrolyte consisting of water, from one to four pounds of chromium trioxide per gallon of water, and from one to five ounces of boric acid per gallon of water, passing a current of from to 250 amperes per square foot of article surface through the electrolyte with the article as an anode, and maintaining the temperature of the electrolyte between and 185 F.

5. In a method of removing a copper, nickel or chromium coating from a ferrous composition base, the step of immersing the coated base in a solution consisting of water, approximately three ounces of boric acid per gallon of water, and about two pounds of chromium trioxide per gallon of water, passing a current of around 200 amperes per square foot of article surface through said solution with the article as an anode, and maintaining the solution at a temperature of about F.

EARL C. HALL. 

